1. Field of the Invention
This invention relates to polymeric metal oxide materials and their formation from metal alkoxide solutions and to articles including such polymeric metal oxides especially in the form of thin films. The invention also provides a composition for use in forming such a polymeric metal oxide.
2. Description of the Prior Art
Various methods are known for producing metal oxide films. Many of these methods do not produce a polymeric metal oxide. A polymeric metal oxide film is a ceramic material which it is desirable to produce without a firing step, since the high temperature of firing makes it impossible to form the film directly on a substrate which has low heat-resistance. Particular uses of such oxide films are as insulation films and corrosion resistant films.
Known methods of forming an oxide film include the sputtering method, which is a physical film-forming method, such as described on pages 1362 to 1366 of Journal of Vacuum Science Technology A (J. Vac. Technol. A), Vol. 1, No. 3 (1985) and the chemical vapor deposition process (CVD process), which is a chemical film-forming method, such as described on page 927 of Journal of Electrochemical Society (J. Electrochem. Soc.), 120 (1973).
The sputtering method includes a high-frequency sputtering method using an oxide as the target, a method of forming a metal thin film while using a metal as the target and thermally oxidizing the film, and a method of forming an oxide thin film by reactive sputtering (using Ar+O.sub.2 as the sputtering gas).
The CVD process includes a thermal CVD process using a metal chloride as the raw material, and an optical CVD process aiming at film formation at a lower temperature such as is disclosed in JP-A-61-190074 (1986).
Further, there is also known, as a chemical film-forming method, the sol-gel method, in which growth occurs in liquid. Although long known for production of for example porous silica, the sol-gel method, such as described on pages 725 to 730 of Material Research Society Symposium Proceedings (Mat. Res. Symp. Proc.) 73, (1986) has recently attracted attention as a low-temperature oxide film-forming method. Reference is also made to JP-A-62-97171 (1987) and JP-A-53-149281 (1978).
There is also known a film-forming method of reacting various silane compounds containing a light activating catalyst such as .alpha.-hydroxy ketone so as to produce a reticulate structure having three-dimensional crosslinks, and hardening the reticulate structure by increasing the crosslinks by light radiation (pages 429 to 436 of the XIVth International Congress on Glass (XIV Intl. Congr. on Glass) 1986).
Some disadvantages of these known methods are as follows.
Of the above-described methods, the sputtering method is performed under a high vacuum, and produces a film having many oxygen defects, thereby making it impossible to obtain a film having a stoichiometrical composition or even close to such a composition. In addition, argon gas and the like which are used as the sputtering gas tend to remain in the film, and the oxygen defects and the residual gas exert deleterious influence on the property of the thin film.
The thermal CVD process requires a high temperature, such as not lower than 600.degree. C., in order to obtain a metal oxide film by the hydrolysis of a metal halogenide, which is a raw material.
The optical CVD method is a method of forming a film at a lower temperature by utilizing light energy for the decomposition reaction of a raw material. It is, however, impossible that the light energy provides all the reaction energy for the decomposition of the raw material and the reaction of the decomposed raw material with oxygen, and it is necessary to provide heat and to heat the substrate. Although this method increases the growth rate of an oxide thin film, the film quality is equal to that of the film obtained without radiation of light.
In the method of thermally oxidizing a film after the sputtering or CVD process, a heating step is essential, so that it is difficult to form a film on a substrate having a low heat resistance such as an organic substrate, or a substrate having a large difference in the thermal expansion coefficient compared with the film. Both the sputtering method and the CVD process require a large-scale vacuum apparatus. The apparatus is expensive and the formation of a film on a large area is difficult.
The sol-gel method is a method of synthesizing an inorganic polymer, which is a ceramic material, at a temperature approximate to room temperature by a chemical reaction in a solution. However, since a metal alkoxide, which is an organic compound, is used as a raw material, carbon tends to remain in the product. In addition, the reaction takes an inconveniently long time. Although a step of heat treating after film formation is known, the heating produces a problem if the substrate has low heat resistance. Even if ultraviolet radiation is used on the film, it is not possible to avoid large amounts of residual carbon.
The method of radiating light on various silanes containing light activating catalyst aims at increasing three-dimensional crosslinking, and removal of an organic substance from the film is not taken into consideration.
In the light of the present invention described below, some comments on some of the specific prior art items mentioned above and some others can usefully be made.
In the method of JP-A-61-190074 (1986) ultraviolet light is directed at the alkoxide molecules M(OR).sub.n (where M is a metal) which are present in the gas phase in a CVD process, to accelerate decomposition of the alkoxide to the metal oxide. The metal oxide molecules deposit on a substrate in an unpolymerized state. Thus the film produced is not polymeric.
JP-A-62-97151 (1987) describes a sol-gel method of forming a recording film of an optical information storage disc. The film is made of Sb.sub.2 O.sub.3. Light is not used in the film formation.
JP-A-53-149281 (1978) discloses a sol-gel process in which a monomer, tetramer or octamer of Ti(OR).sub.4 is dissolved in hexane and isobutylalcohol. The solution is coated on a thin polyester film and hydrolysis of the alkoxide is performed to create a polymeric oxide gel. The gel is then subjected to irradiation with ultraviolet light (from a Hg lamp) to produce a hydrophilic film. This irradiation step effects decarbonization of the previously formed gel.
In JP-A-64-87780 (1989) there is described a process in which similarly an alkoxide solution is coated on a substrate, dried and then irradiated to reduce the carbon content.
JP-A-1-294535 (published 28 Nov. 1989) is concerned with the production of a superconductor powder. Bi(OR).sub.3, Sr(OR).sub.2 and Cu(OR).sub.2 are dissolved in isopropanol containing water, and refluxed to remove the isopropanol and obtain a solid mixture. The solid mixture is irradiated with ultraviolet and infra-red radiation, to obtain the desired mixed oxide powder.